This binding is similar to mutt's, which is
bind {mode} b "bounce-message" # remail a message to another user
where {mode} is 'index', 'pager' or 'attach'.
(define-key map (kbd "TAB") 'notmuch-show-next-button)
(define-key map "f" 'notmuch-show-forward-message)
(define-key map "F" 'notmuch-show-forward-open-messages)
+ (define-key map "b" 'notmuch-show-resend-message)
(define-key map "l" 'notmuch-show-filter-thread)
(define-key map "r" 'notmuch-show-reply-sender)
(define-key map "R" 'notmuch-show-reply)